J . Am. Chem. SOC.1984, 106, 5913-5920
5913
Synthesis, Characterization, and Equilibria of Palladium(11) Nitrile, Alkene, and Heterometallacyclopentane Complexes Involved in Metal Nitro Catalyzed Alkene Oxidation Reactions Mark A. Andrews,* Tony C.-T. Chang, Chi-Wen F. Cheng, Thomas J. Emge, Kevin P. Kelly, and Thomas F. Koetzle Contribution from the Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973. Received December 15, 1983
Abstract: Treatment of tran~-Pd(CH$N)~Cl~ with AgN02 in acetonitrile gives c ~ s - P ~ ( C H , C N ) ~ C ~(1) N that O ~ partially dissociates in chloroform solution to give [Pd(CH3CN)C1NO2I2(2). Alkenes react with 1 or 2 to give the alkene nitro complexes [Pd(alkene)C1N0212(3), the cis-cyclccctene member of which has been isolated and fully characterized. Complexes 3 subsequently undergo internal cyclization by nucleophilic attack of a nitro oxygen atom on the coordinated alkene to give heterometalla, cyclopentane (chelated alkyl nitrite) complexes [Pd(N(=O)OCR,CR,)Cl], (4), in which the alkyl substituents (if any) are preferentially located on the carbon fi to Pd. Representative examples of these metallacycles have been isolated, and complex 4g derived from the substituted norbornene dicyclopentadiene has been characterized by a single-crystal X-ray structure crystallizes in the space group Pbca with a = 24.717 (6) A, b = 9.733 (2) A, determination. Pd2C12(CloH12N02)2~CH2Cl~ c = 20.655 ( 5 ) A, V = 4969 (1) A3, and 2 = 8 at 120 K. The final discre ancy indices were R l ( F o )= 0.062 and R,(Fo) = 0.077. The Pd-CI bond distances trans to carbon (2.499 (4) and 2.491 (4) are longer than those trans to nitrogen (2.331 (4) and 2.328 (4) A). The Pd-N distances (1.94 (1) and 1.95 (1) A) are among the shortest reported to date. Complexes 1-4 are all in rapid (often NMR time scale) equilibrium with each other under the reaction conditions. (Similar behavior is observed for the previously known dichloro analogues of complexes 1-3.) Although quantitative data regarding these equilibria are not obtainable, qualitative kinetic and thermodynamic observations are presented that are relevant to the extensive Pd(I1)-mediated chemistry of alkenes. Metallacycles 4 are also involved in bridge cleavage and possibly ring-opening equilibria.
1)
Background For over 10 years, a wide variety of metal-mediated 0 atom transfer reactions have been suggested to proceed via heterometallacyclic complexes, but very little concrete evidence for such intermediates has been forth coming.14 For example, Mimoun and others have proposed that heterometallacyclopentanes i and ii are responsible for the oxidation of alkenes by metal dioxygen and alkyl peroxide complexes, respectively.’ The only such species that have been observed, however, are derived from highly elec( I ) For leading references see: (a) Mimoun, H. Angew. Chem., Int. Ed. Engl. 1982, 22,734-750. (b) Mimoun, H. J . Mol. Catal. 1980, 7 , 1-29. (c) Sheldon, R. A,; Kochi, J. K. “Metal-Catalyzed Oxidations of Organic Compounds”; Academic Press: New York, 1981. (d) Di Furia, F.; Modena, G. Pure Appl. Chem. 1982.54, 1853-1866. ( e ) Minoun, H.; Seree De Roch, I.; Sajus, L. Tetrahedron 1970, 26, 37-50. (f) Arakawa, H.; Moro-Oh, Y.; Ozaki, A. Bull. Chem. Soc. Jpn. 1974, 47, 2958-2961. (g) Read, G.; Walker, P. J. C. J. Chem. SOC.,Dalton Trans. 1977,883-888. (h) Read, G. J . Mol. Catal. 1978, 4 , 83-84. (i) Tang, R.; Mares, F.; Neary, N.; Smith, D. E. J. Chem. Soc., Chem. Commun. 1979,274-275. Purcell, K. F.J . Organomel. Chem. 1983, 252, 181-185. (k) Lyons, J. Aspects Homogeneous Catal. 1977, 3, 1-136. (2) (a) Sheldon, R. A.; Van Doorn, J. A. J . Organomet. Chem. 1975,94, 115-129. (b) Tatsuno, Y.; Otsuka, S. J . Am. Chem. SOC. 1981, 103, 5832-5839. (c) Halfpenny, J.; Small, R. W. H. J . Chem. SOC.,Chem. Commun. 1979,879-880. (d) Igersheim, F.; Mimoun, H. Nouu.J . Chim. 1980, 4, 161-166. (e) Note Added in Proof A metallocyclic intermediate has recently been characterized in the reaction of Pt(PPh3)202with (E)-2-butenal: Broadhurst, M. J.; Brown, J. M.; John, R. A. Angew. Chem., Znt. Ed. Engl. 1983, 22,47-48. (3) (a) Efraty, A,; Arneri, R.; Sikora, J. J. Organomet. Chem. 1975, 91, 65-70. (b) Ileperuma, 0. A.; Feltham, R. D. J. A m . Chem. SOC.1976, 98, 6039-6040. (c) Doughty, D. T.; Gordon, G.; Stewart, J. P., Jr. J . Am. Chem. Soc. 1979,202,2645-2648. (d) Doughty, D. T.; Stewart, R. P., Jr.; Gordon, G. J. Am. Chem. SOC.1981, 203, 3388-3395. (e) Kriege-Simondsen, J.; Bailey, T. D.; Feltham, R. D. Znorg. Chem. 1983, 22, 3318-3323. (f)For a comprehensive bibliography of 0 atom transfer reactions of metal nitro complexes see: Andrews, M. A,; Chang, T. C.-T.; Cheng, C. F. Organometallics, in press. (4) Possible spectroscopic evidence for a metallacyclic intermediate in 0 atom transfer to CO in [Fe(PPha)2(CO)3(N02)]*has been reported (Baker, P. K.; Broadley, K.; Connelly, N. G. J . Chem. SOC.,Chem. Commun. 1980, 775). Notably absent, however, were any spectroscopic features attributable to the metallacyclic functionality itself, e.g., uc4 and uN4.
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0002-7863/84/1506-5913$01 .50/0
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i. x = O ii. X = O ; Y = H , alkyl iii. X = N ; Y = O
tron-deficient alkenes or from dIo transition metals.2 Similarly, heterometallacyclobutanes iv have been proposed as intermediates r
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iv
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in metal nitro mediated oxidations of CO, C S , and NO (eq l).3 In this case, no fully characterized model compounds have been rep~rted.~ In the course of extending nitro group 0 atom transfer to other substrates, we have found that cis-bis(acetonitri1e)chloronitropalladium(I1) (1) effects the oxidation of alkenes via readily detectable alkene and type iii heterometallacyclopentane comp l e ~ e s . We ~ report here details of the synthesis and characterization of representative examples of each of these compounds, together with qualitative observations concerning the facile and multiple equilibria which link them (Scheme I). Results Catalyst Preparation and Characterization. Yellow cis-bis(acetonitrile)chloronitropalladium(II) (1) is readily prepared in a one pot synthesis by dissolving commercial palladium dichloride (5) (a) Andrews, M. A.; Kelly, K. P. J. Am. Chem. SOC.1981, 103, 2894-2896. (b) Andrews, M. A.; Cheng, C.-W. F.J. Am. Chem. SOC.1982, 104, 4268-4270. (c) Andrews, M. A,; Chang, T. C.-T., Cheng, C.-W. F.; Kelly, K. P. Organometallics, in press.
0 1984 American Chemical Society
5914 J . Am. Chem. SOC..Vol. 106, No. 20, 1984
Andrews et al.
Scheme I'
nificant changes in the vibrational frequencies of the nitro group indicates that dimer formation occurs through chloride bridging, CH3CN \pd/c' not nitro bridging.* / \ Room-temperature 'H N M R spectra of solutions of nitro CH3CN NO2 complex 1 in CDC1, (which contain monomer I, dimer 2, and free 1 2 acetonitrile) show only one singlet at 2.19 ppm resulting from dynamic averaging of the free and coordinated acetonitrile present. At -56 OC, the exchange rate is slow on the N M R time scale and four peaks are observed at 2.53, 2.48, 2.43, and 2.12 ppm, the first three due to coordinated nitrile and the latter to free nitrile. (The nitrile chemical shifts are significantly temperature dependent, free acetonitrile having a chemical shift of 2.00 ppm at room temperature.) The equal intensity peaks at 2.53 and 2.43 ppm are tentatively assigned to the two types of methyl groups 3 in monomer 1 while the larger peak at 2.48 ppm is assigned to the methyl groups in dimer 2." The monomer4imer equilibrium constant KD is clearly temperature dependent favoring the monomer at lower temperatures. The mixed chloro nitro complexes 1 and 2 appear to be relatively stable toward disproportionation since only a small peak due to the dichloro complexes is seen in 'H N M R spectra of solutions of 1. I; 6 Alkene Nitro Complexes 3. The palladium alkene complexes 5 [Pd(alkene)C12]2,though of limited stability, are readily prepared from bis(benzonitri1e)palladium dichloride.I2 The reactions of %,R'-R4 = H ; b . R ' = Me; c , R ' = R 2 = M a : d , R ' , R 3 = (CH2)6; nitro complex 1 with alkenes are analogous, except for th: added o , R'. R3 = f,R',R3=&;g,R',R3= complication of subsequent facile metallacycle formation (vide infra). Addition of most alkenes to CDC13 solutions of 1 leads to partial displacement of acetonitrile by alkene as evidenced by h . R', R3 = an instantaneous color change from orange to yellow and the appearance of a second C = C stretching frequency in the infrared at 100 cm-' below that of the free alkene. In the IH NMR, in acetonitrile to give trans-bis(acetonitrile)dichloropalladium(II)~ the dynamically averaged acetonitrile methyl resonance shifts followed by in situ treatment with 1 equiv of silver nitrite. The upfield due to the higher fraction of free acetonitrile now present. nearly equal intensities of the two methyl C-H stretching modes The alkene vinyl and allyl resonances shift downfield with free in solid-state infrared spectra of 1 are indicative of a cis geometry and coordinated alkene also exchanging on the NMR time s ~ 3 l e . l ~ for the acetonitrile ligands.6b This is also supported by the obThis exchange is difficult to freeze out for most alkenes, even at servation of a very slight splitting of the C N stretch in Pd(CD3CN)2CIN02which is not seen in ~ ~ u ~ ~ - P ~ ( C D , C(InN ) ~ C -80 ~ ~ .OC. The only alkene nitro complex that we have been able to isolate the protio species the C N stretch is mixed via Fermi resonance from these solutions and fully characterize is dimer 3d derived with a liCH+ vm combination mode.' The vibrational frequencies from cis-cyclooctene. Most other alkenes investigated bind less assigned to the nitro group ((mull spectra): 1451, 1335, 826, and strongly to Pd(II), are more rapidly oxidized via metallacycles 600 cm-I, confirmed by I5N and l 8 0 labeling) clearly show that 4, or both. Aside from its unusual stability (no decomposition the nitro group is terminally N bonded.8 These frequencies are in dry air for over 2 weeks at room temperature), alkene nitro only slightly shifted in CD3CN (1460, 1338 cm-l) and CDCl, complex 3d appears to be quite typical. The C=C stretch is (1478, 1315 cm-') solution spectra. While the former solutions observed (CDCl, solution) at 1525 cm-' (vs. 1653 cm-' for free are yellow as expected, the latter are orange and exhibit infrared cis-cyclooctene and 1514 cm-' for the analogous dichloro complex). peaks attributable to free acetonitrile, consistent with formation The nitro group vibrations are observed at 1469, 1312, and 8 19 of dimer 2 (KD, top of Scheme I). Similar behavior has been cm-I. In the 'H NMR, the vinyl protons in 3d appear as a quintet observed for Pd(RCN)$12 c o m p l e x e ~ .The ~ ~ ~absence ~ of sigcentered at 6.16 ppm (vs. 5.62 ppm for free cis-cyclooctene and 6.20 ppm for the dichloro analogue), while the allylic protons (6) (a) Walton, R. A. Spectrochim. Actu 1965, 21, 1795-1801. (b) resonate at 2.31 ppm (vs. 2.12 and 2.31 ppm for free alkene and Walton, R. A. Can. J . Chem. 1968, 46, 2347-2352. (c) Wayland, B. B.; dichloro analogue, respectively). Schramm, R. F. Inorg. Chem. 1969,8, 971-976. (d) Hegedus, L. S.; WilHeterometallacycles 4. Alkene nitro complexes 3 undergo liams, R. E.; McGuire, M. A,; Hayashi, T. J . Am. Chem. SOC.1980, 102, 4973-4979. internal cyclization to give equilibrium concentrations of heter(7) We believe that the literature assignments6* for the vCN and the 6cH ometallacyclopentanes 4 (middle of Scheme I) in a period of + vcc combination mode in Pd(CH,CN)2C12should be reversed. We attribute seconds (i~obutylene'~ and norbornene) to minutes (ethylene and the reversal to an incorrect assignment of the vcc mode which we assign to propylene). Although these species are usually readily detectable a sharp weak peak at 955 cm-I (shifting to 867 cm-' on deuteration) rather than 970 cm-I as given in the literature. by spectroscopic means, isolation and unambiguous characteri(8) Hitchman, M. A.; Rowbottom, G. L. Coord. Chem. Rev. 1982, 42, zation were only possible in the case of bicyclic alkenes. Internal 55-132. . . and cyclic alkenes have very low metallacycle formation constants (9) Kitching, W.; Moore, C. J.; Doddrell, D. Inorg. Chem. 1970, 9,
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541-549. (10) For Pd(RCN)2CI2 (R = Me, PhCH2) complexes, IH NMR spectra in CDC13 show that partial nitrile dissociation occurs but, in contrast to Pd(MeCN),CINO,, free and coordinated nitrile do not exchange on the NMR time scale at room temperature. Over a period of -3 h, some PdC12 is observed to precipitate until a steady-state equilibrium is achieved. By integrating free vs. coordinated nitrile for the more soluble phenylacetonitrile complexes and weighing the PdCI, formed in four samples with different [Pd(II)], and [RCN],, approximate dissociation constants at 27 "C were obtained: Koi = [(PdLC12)2][L]2/[PdL2C12]2 0.10 0.05 M, KDz [LI2/[(PdLCl2),] = 0.8 0.3 M. The large errors are due to the magnified effect that small experimental errors have when taking small differences between large numbers. Qualitatively, the following order of ligating strength is observed for weak ligands toward PdCI2: HCONMe,
